This disclosure relates to non-contact electrical testing of printed circuit boards, solid state display devices, integrated circuits and other substrates carrying traces that connect together components of a circuit.
The goal of such non-contact testing is to find excessively low leakage resistance between different interconnection traces, between any one trace and the ground plane, and excessively high resistance along an individual trace. It is desirable to find circuit malfunctions caused by interconnection defects early in the electronic manufacturing process to minimize the cost of repairs and to maximize the yield.
Conventional network prober systems inject alternating current (hereinafter AC) or direct current (hereinafter DC) into the device under test (DUT). Conventional capacitively coupled probers can inject only alternating current. However, both techniques must necessarily make mechanical contact between test probes and at least two points in the DUT in order to establish a flow of current. The small size and high density of present day interconnection elements make it difficult to make reliable contact in all circumstances, and the physical contact may damage the DUT. As interconnect traces become smaller and more densely packed, the test cycle for an entire DUT becomes very long because testing of each point requires mechanical motions of the test probe(s). Thus faster, non-contacting methods are needed to economically test more advanced DUTs.
Known voltage contrast methods for such testing use light or particle beams that may be focused and scanned very quickly from test point to test point, thus reducing the total time required to inspect a circuit. These methods induce current signals by illuminating the test point with light, electron or ion beams. These test systems scan the beam very rapidly from test point to test point using optical, electrostatic or magnetic deflection. Most of the beam energy incident on a test point is absorbed, causing the circuitry to charge negatively where the beam is an electron beam. Secondary electrons are ejected from the DUT surface, and are detected to measure the voltage of the DUT surface as it charges. The difference between absorption and secondary emission current determines the net rate at which the DUT surface charges. Secondary electrons leave the surface with low average energy, and gain or lose additional energy depending upon the potential of the test point DUT surface with respect to its surroundings. An electron leaving a negatively charged area gains more energy than one leaving a positive area because negative charge repels electrons. A detector which measures current depends upon energy as well as number of secondary electrons can detect surface potential at the point illuminated by the beam. The prior art is replete with such techniques using a beam to test for electrical properties of interconnection network elements in a DUT.
U.S. Pat. No. 4,417,203 relates to non-contact testing of three dimensional networks of conductors embedded in dielectric material. The system described uses two beams; a flood beam and a focus probe beam. The flood beam applies a negative charge to either the top or the bottom surface of the specimen and then the probe beam scans the network generating secondary electron emission, which subsequently is processed by filtering and digitizing. U.S. Pat. No. 4,843,330 also shows a system that uses two beams, a flood beam and a focus probe beam. In addition it also illustrates the use of a biasing grid to enhance the detected voltage contrast.
Another prior art electron beam tester, see U.S. Pat. No. 5,834,773, uses one beam in combination with deflection plates that direct electrons from the substrate onto a smaller but similarly biased detector. The secondary electrons are steered into the detector by low voltage deflection plates driven in synchronization with beam position. This arrangement makes it possible to observe widely spaced test points, and also allows the use of a solid state or electron multiplier pre-amplifier to minimize detection noise.
One form of prior art voltage contrast detector places a biased grid above the substrate, so that only electrons with energy above a certain threshold may pass through and reach a second more positively biased collector. The grid may also be enclosed in a low bandwidth servo loop that varies grid voltage to maintain constant detected current. Using this method, the grid voltage instead of the detected current records the surface potential. However, detectors with a grid next to the substrate are not advantageous for wide field electron beam testers because it can occlude the beam.
Prior art systems also use two beams to test one side of a substrate in a serial manner, one beam performing a test while the other is being deflected to a new test site. This method reduces the time lost to deflection overhead, but does not employ two beams to simultaneously observe interconnected test points. Using only one beam, direct measurement of impedance between two test point is not possible, but such methods can detect leakage resistance of 200 Mohm or less and series resistance under 2 Mohm. A much lower threshold for measuring series resistance is desirable. It is also desirable to be able to better discriminate, than in the prior art methods, between the various possible defects in the interconnection network. A test time shorter than the prior art method is desirable to reduce the cost of testing.
The present disclosure relates to an apparatus and a method of non-contact electrical testing of printed circuit boards, solid state display devices, integrated circuits and other DUTs having traces that connect together components of a circuit, using two modulated charged particle beams in a uniform axial magnetic field environment.
The apparatus includes two charged particle sources each generating one beam, electrodes to modulate the beams, optics to focus the beams and deflection coils to deflect the beams over a large area. The apparatus has an enclosure for the optics and the deflection coils. The enclosure is made of magnetically soft material. A solenoid excitation coil creates an almost uniform axial magnetic field within the magnetic enclosure. A detection system for detecting the voltage contrast signals, including the signal processing system is also included.
The uniform magnetic field results in reduced axial aberration, deflection aberration and spot growth in the beams, this in turn results in the optics providing higher beam current from a source of given brightness.
The associated method using two beams allows a fast, direct measurement of impedance parameters of an interconnection network on a DUT. The use of modulated beams and frequency filtering of the voltage contrast signals allows such measurement to be quantitative, more sensitive and more discriminating between various possible defects in the interconnection network then the prior art methods. By selecting appropriate beam modulation frequencies the sensitivity to a certain kind of defect is selectively increased, and by combining DC measurement techniques with the present AC method the range of measurement of leakage is improved. In particular, a lower threshold for measuring series resistance is achieved by the present invention.